why is it significant that Don Pedro and his men are returning from the wars?

Well for one reason a war or battle has successfully been completed. Now, given that this is a romantic comedy, their thoughts can turn to love, sex, courting....This also gives the conniving Don John a way to get back at his brother Don Pedro. War also lingers on in romantic terms with the characters of Beatrice and Benedict. They constantly squabble which becomes a war of the sexes.

Their return from war is quite signficance as this will ensure they have time to concentrate on theirt personal lives as well as building on their love lives and reconciliation.